The core-hole excitation spectrum of benzene: A symmetry-adapted CNDO/S equivalent-core study including ‘‘spin-symmetry breaking’’ configurations

Abstract

The core-hole excitation spectrum of benzene below 12.0 eV is calculated within a symmetry-adapted first-order configuration interaction approach based on the CNDO/S equivalent-core approximation. One-electron singlet excitation energies of the neutral molecule are shown to approximate the corresponding ‘‘singlet’’-coupled doublet energies in the delocalized core-hole species. Intensities in accord with experiment are obtained within the sudden approximation by projecting the C2ν reduced-symmetry equivalent-core solutions out of the first-order delocalized description. The total core-hole excitation intensity below 12.0 eV is found to be largely attributable to three one-electron π*←π excitations of singlet-coupled doublet character which are split into the observed number of features, with correspondingly observed strengths, by first-order configuration interaction with energetically degenerate excitations in the ‘‘triplet’’-coupled doublet manifold—‘‘spin-symmetry breaking configurations.’’ The observed core-hole excitation satellite at 10.7 eV and the asymmetry on the high binding energy side of the well-resolved satellite at 8.3 eV are interpreted in terms of two excitations of 1E2g (π*←π) neutral molecule parentage. The assignment of these higher-energy features as 1E2g(π*←π) in origin provides further support for the numerous theoretical predictions and recent conclusions based on spectroscopic investigations for states of 1E2g symmetry at approximately 8.0 and 10.0 eV in neutral benzene.